The present invention relates to a hydraulic brake system with slip control.
The discontinuous pressure fluid control in slip-controlled brake systems by way of digitally operated inlet and outlet valves produces an undesirable emission of sound due to the pulse-like pressure variation.
Therefore, various arrangements to reduce noises during anti-lock/traction slip control operations have been disclosed. In this respect, reference is made to international patent application WO-A-90/12713, as an example. This application discloses the arrangement of pressure pulsation dampers inside the main pressure lines, and for that matter, in the vicinity of the braking pressure generator, or in the by-pass line of the electromagnetic inlet and outlet valves. The pressure pulsation dampers used are configured as vibration-damping elastomeric energy-accumulating elements which, by way of their defined expansion in volume, flow length and restricting properties, shall prevent the transmission of sounds produced by valve switching frequencies to the mass-loaded and, thus, vibrating brake system.
A relatively soft pedal feel and an increased pedal travel are undesirable consequences of the additional pressure fluid volume input caused by the given pressure pulsation dampers during braking operations.
European patent application No. 0 317 305 discloses a solenoid valve which is appropriate for use in anti-lock hydraulic brake systems. The solenoid valve includes a magnetic core accommodating a coil which is confined by a magnetic armature on one side and by a diaphragm member on the other side. The diaphragm member has a passage closable by a valve needle. The diaphragm member is axially movably arranged between the magnetic core and a housing cover containing the pressure fluid inlet. Thus, in the switch position of the valve needle closing the supply channel in the diaphragm member, a pressure differential is operative on both sides of the diaphragm member and causes a displacement of the valve needle relative to the magnetic armature. This fact preloads a compression spring compressed between the valve needle and the magnetic armature. The preloading force of the compression spring, caused by the difference in pressure on the diaphragm member, causes rapid release of the magnetic armature from the magnetic core when the electromagnetic energization is interrupted. The result is short valve opening times. A spring restoring force acting on the valve needle in the opening sense assists the rapid opening of the supply channel in the diaphragm member so that pressure fluid, after having passed the open passage across the valve needle, propagates to an annular slot which is formed between the hollow cylindrical inside wall of the magnetic core and the outside wall of a cylindrical part which guides the valve needle. The fluid emanating from the valve inlet is conducted to a pressure fluid connection leading to the pressure fluid consumer by way of the supply channel in the diaphragm member and the subsequent annular slot.
UK patent application No. 2 252 140 discloses a valve assembly for anti-lock control including an inlet valve and an outlet valve. The inlet valve accommodates an annular piston adapted to be actuated in response to hydraulic pressure differences. The piston is configured as a stepped piston which is pressure-balanced, subjected to the hydraulic pressure of the master cylinder. A compression spring interposed between the valve carrier of the electromagnetically operable inlet valve and the annular piston keeps the annular piston in an unrestricted open position, with the result that pressure fluid is unimpeded to flow from the master cylinder to the wheel brake. When the inlet valve is energized electromagnetically, the valve closure member of the inlet valve adopts its closed position, and the electromagnetically energized outlet valve discharges the wheel braking pressure in the direction of a supply reservoir. This reduces the level of wheel pressure which is conducted from a branch line of the outlet valve to a piston step on the annular piston. Thus, the annular piston is no more pressure-balanced and, with its sealing seat, closes the originally unimpeded pressure fluid passage in the inlet valve from the master cylinder to the wheel brake in opposition to the effect of the compression spring. A new pressure increase in the wheel brakes occurs by pulsed operation of the magnetic coils of the inlet valve so that pressure fluid propagates in a restricted fashion to the wheel brake by way of the orifice bore in the annular piston. To this end, the outlet valve is in an operating position in which the connection to the supply reservoir is closed. Upon deactivation of the master cylinder, the annular piston adopts again a position in which the pressure fluid connection to the wheel brake is unrestrictedly open because the compression spring positions the annular piston on its stop.
The chosen channel arrangement and pressure fluid ducts to provide a connection between the master cylinder, the inlet and outlet valve and the wheel brake necessitates a structural series arrangement of the individual elements in the inlet valve. Also, the selected switching function of the annular piston in response to the amount of pressure conducted from the wheel brake by way of the outlet valve necessitates the chosen arrangement.